As far as we can ascertain, the type of refraction technique outlined in the abstract has never been practiced before. The fact that porous subsurface geological formations containing a gas phase in at least some of the pores had a different acoustic response from an equivalent, liquid filled formation and that in fact, the attenuation of seismic waves traversing such formations was different, has been known for some time. A recent paper by Dr. S. N. Domenico (Geophysics, Vol. 42, No. 7 (December, 1977), pp. 1339-1368) both gives research results and an adequate discussion of the prior literature. One can at least say that in the 1950's it was known that the velocity and attenuation of seismic waves in petroleum-bearing porous formations was to a degree different from that for those filled only with connate water. Since this changed the reflection coefficient between a reservoir bed and an adjacent bed, the so-called "bright spot" reflection tecnhique was initiated, in which attention was paid not only to the travel time of reflected seismic waves but (and almost uniquely) to the amplitudes of the reflected seismic waves. If a seismic reflection received at a number of different geophone group locations showed that in a region the amplitudes significantly decreased and if it were known that that particular formation was capable of being a reservoir bed, investigators at least felt it merited drilling to determine, for example, the presence of natural gas. It was soon learned that the technique was more qualitative than quantitative since the major change in amplitude of such seismic reflections mostly occurs between a completely liquid filled reservoir and one containing something of the order of 5 to 10% gas saturation. Beyond this, the change is relatively small and hence one has little correlation of the degree or extent of the gas reservoir present. This could be summarized by the statement in the cited reference that "determination of brine saturation (and thereby quantity of gas) from variation in reflection amplitude would be extremely difficult." No mention is made in this paper (nor, as far as we can find out, in the references listed in the extensive bibliography) referring to refraction characteristics.
Refraction seismometry is of course nowhere near as popular as reflection seismometry. The only modern book which considers refraction prospecting in considerable depth is the volume "Seismic Refraction Prospecting," published in 1967 by the Society of Exploration Geophysicists under the editing of A. W. Musgrave. There is a section in this volume (pp. 85-193) dealing with the amplitudes of refraction signals. The most pertinent material here is in the first paper by P. M. S. O'Brien, "The Use of Amplitudes in Seismic Refraction Survey" (pp. 85-118). O'Brien teaches a method of correcting the measured initial amplitudes of received refracted waves for variations due to nongeologic causes, which includes the angle of approach of the incident wave, the ground in the close vicinity of the geophone and the geophone "plant," and the observing system (which well could be called instrumentation), that is the overall sensitivity from the plant through the geophone, the amplifier, and recorder to the measured or recorded amplitude. Applying these corrections, according to O'Brien, is possible, " . . . However, a proper analysis cannot be done to routine; it requires the skill and care of a competent seismologist. Not because the basic concepts are particularly complicated but because the relative importance of the various factors involved may often be difficult to evaluate." He then points out that once allowance has been made for the purely geometrical attenuation due to enlargement of the wave front, instrumentation, and any variation in charge weight, the residual values will depend on such things as lithology, refractor thickness, structure, etc. The closest he comes to discussing attenuation in terms of matters important in our invention lies in his section referred to as Basement Identification, pointing out that after the coefficient of attenuation has been converted from per unit distance into attenuation per predominant wave length, it will probably be found that those having an attenuation of several tenths of a decibel per wave length along the refractor indicate a thick, porous refractor. He mentions nowhere the effect of difference in fluid content of the refractor. He does not refer to the beds lying between the refractor and the geophone groups, which is basically the region of interest in the method we outline below. He also has no comments whatsoever about the pronounced differences that can exist using a technique involving short-range and long-range refractions profiles, which is a basic necessity in our method.
The only other reference found which even by inference considers the matter of gas saturation in a subsurface zone in the course of refraction surveying is in the relatively modern text "Interpretation Theory in Applied Geophysics" by F. S. Grant and G. F. West, McGraw-Hill Book Company, 1965, pp. 153-155. Here the authors state that difficulties arise in refraction interpretation when the velocity does not increase monotonically with the depth. They then show in FIGS. 5-17, page 154, the plotted arrival time of the head wave from a refraction profile at varying distances between shot point and center of spread when the subsurface contains a bed which is characterized by a decreasing velocity. While they go no further, we recognize that such variation is possible in a shallow, gas-containing zone. The authors (in common with the vast majority of those writing on refraction profiling) deal only with arrival times of head waves or so-called first breaks in refraction surveying, and do not deal with amplitudes at all.
There have been some instances of vertical seismic profiling when access to wells penetrating the desired subsurface formations is available. In this case a number of well geophones are suspended on a suitable multiconductor cable so the individual geophones may be recorded. Dynamite charges are then detonated at or near the surface and the first breaks of the well geophones recorded. If there is a gas zone in the ray path from source to geophone, an abnormally low amplitude is recorded. However, such wells are usually not available and even if such can be used, there is only a relatively small zone around the well that can be investigated by this method.
It should be therefore apparent that there seems to be no prior art relatively close to our technique.